The present invention is directed to probe stations particularly suitable for making highly accurate low-current and low-voltage measurements of electronic devices, and especially electronic devices fabricated on a wafer. More particularly, the present invention relates to additional layers used in conjunction with a temperature-controlled chuck within such a probe station.
Integrated circuit devices are typically manufactured in and on a single wafer of semiconductor material using well-known techniques. Prior to cutting the individual integrated circuit devices from the single wafer for encapsulation, typically predetermined test sequences are run on each integrated circuit device on the wafer to determine if each device operates properly. A probe card, which includes a plurality of electrodes configured to conform to the particular geometry of the integrated circuit devices fabricated on the wafer, may be used in conjunction with a probe station to test the circuitry. The wafer and probe card are moved relative to each other until all the integrated circuit devices on the wafer have been tested. Schwindt et al., U.S. Pat. No. 5,345,170, Harwood, et al., U.S. Pat. No. 5,266,889 and Schwindt, et al., U.S. patent application Ser. No. 08/100,494 filed Aug. 2, 1993, disclose examples of probe stations with which this invention may be used, and are incorporated herein by reference. Alternatively, individually positionable probes may be used for testing wafers and other types of test devices in conjunction with a probe station.
Many integrated circuit devices are designed to operate at temperatures other than ambient room temperature. To accommodate device testing at nonambient temperatures, temperature-controlled chucks, generally referred to as thermal chucks, may be employed. One design of a thermal chuck includes a base with internal cavities for circulating cold fluids to lower the temperature of the thermal chuck below ambient temperature. Affixed to the upper surface of the base are multiple heating-cooling elements and a conductive top surface. The heating-cooling elements, typically constructed of peltier devices, regulate the temperature of the top surface by varying the current level and polarity of the current. If the desired temperature is above a predetermined temperature, such as the ambient temperature, then the heating-cooling elements regulate the temperature. If the desired temperature is lower than the predetermined temperature, such as the ambient temperature, then the heating-cooling elements, in combination with the fluid flow in the base if significant cooling is desired, regulate the temperature. The thermal chuck may include a temperature sensor located on or in the vicinity of the thermal chuck to sense the temperature. In response to sensing the temperature, the level and polarity of the current provided to the heating-cooling elements is automatically varied to maintain a near constant temperature. For example, when the temperature of the thermal chuck is too low, the current level supplied to the heating-cooling elements is increased to raise the temperature. Contrary, if the temperature of the thermal chuck is too high, the current level supplied to the heating-cooling elements is decreased and the polarity reversed to lower the temperature.
The wafer, with its integrated circuit devices, is secured to the top surface of the thermal chuck to perform tests on the circuit devices at different temperatures. However, for low-voltage and low-current measurements an unacceptably high level of noise is observed in measurements when using the thermal chuck. The temperature control system inherently fluctuates the current level provided to the heating-cooling elements in order to maintain the desired temperature. The fluctuations in the current level and polarity supplied to the heating-cooling elements, which are typically small and very rapid in nature, result in fluctuations in the electromagnetic fields produced by the heating-cooling elements and their associated electrical wiring. The traditional wisdom has been that the changing electromagnetic fields produced by the changing currents within the heating-cooling elements and their associated electrical wiring are somehow electromagnetically coupled into the surface upon which the wafer is placed. This conclusion is further supported by the noise observed while making measurements, which appears to have the same relative magnitudes and timing as the fluctuations in the current level supplied to the heating-cooling elements. Accordingly, the traditional wisdom is that the changing electromagnetic fields are the primary source of the noise observed between the wafer and the thermal chuck when taking measurements. It is highly desirable to minimize the noise to facilitate more accurate measurements.
One way to reduce the noise produced by the electromagnetic fields has been to shield the wafer from the electromagnetic fields by adding additional layers to the conductive top surface of the thermal chuck. The additional layers have included a stacked structure of three layers, including a pair of insulation layers each of which is positioned on either side of an interposed copper layer. A conductive supporting layer for supporting the device under test is positioned on top of the aforementioned three layers. All four layers are secured to the thermal chuck by teflon screws but are not adhered to each other. The copper layer is connected to the guard conductor of a triaxial cable, the thermal chuck is connected to the shield, and the supporting layer is connected to the center conductor. The additional layers, and particularly the grounded thermal chuck, act to shield the test device from the electromagnetic fields. A reduction in the noise has been observed; however, significant noise levels still remain.
A different design to attempt to provide superior shielding of the test device has involved replacing the stacked structure of three layers with a copper foil layer encapsulated in polyamide. The foil is connected to the guard conductor and the supporting layer is connected to the center conductor. A further reduction in the noise level with respect to the aforementioned structure has been observed; however, significant noise levels still remain.
A still further design has involved replacing the three stacked layers with a ceramic layer. The thermal chuck is connected to the guard conductor and the supporting layer is connected to the center conductor. The absence of an interposed conductive layer, however, seems to decrease the shielding from the electromagnetic fields resulting in high noise levels.
Yet another alternative design has involved replacing the three stacked layers with a copper foil layer interdisposed between an insulator layer and a ceramic layer. The three stacked layers are not adhered to one another. The foil is connected to the guard conductor, the thermal chuck is connected to the shield, and the supporting layer is connected to the center conductor. During tests this design appears to be an improvement over all the aforementioned designs for shielding the test device from the electromagnetic fields. However, the noise observed is still unacceptably high for low-current and low-voltage measurements.
What is desired therefore is an additional layer structure for use with a temperature-controlled chuck to significantly reduce the noise observed during low-current and low-voltage measurements.